In some video coding systems, such as coding systems implementing the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) H.264 Standard, the International Organization for Standardization (ISO)/International Electrotechnical Commission (IEC) 14496-10-MPEG-4 Part 10, Advanced Video Coding Standard, or the Joint Collaborative Team on Video Coding (JCT-VC) High Efficiency Video Coding (HEVC) specification, a video coding layer (VCL) includes the video processing functionality of the codec, while a network abstraction layer (NAL) encapsulates the output of the VCL encoder for network transmission.
The video coding layer may include an encoder that generates a coded video sequence that includes a sequence of coded pictures. A coded picture of the sequence may be a collection of one or more slices. For example, the VCL encoder may be a macroblock-based coder that utilizes inter-picture prediction with motion compensation, and transform coding of the residual signal. The VCL encoder may output one or more slices that include a sequence of macroblocks of a picture along with a header that contains information, e.g. overhead information, related to the slice, such as the spatial address of the first macroblock in the slice, the initial quantization parameter, etc. The VCL encoder may also generate parameter sets that contain information pertaining to one or more slices or pictures, such as sequence parameter sets or picture parameter sets. The parameter sets may generally contain information that may be applicable to multiple slices or pictures. The parameter sets may include an identifier that may be used by the one or more slices and/or pictures to reference the parameter sets.
The network abstraction layer may include an encoder that encapsulates the slices into Network Abstraction Layer Units (NAL units), that may be transmitted over a network, such as over a packet network. A NAL unit may include a header, separate from the headers of the slices. Thus, the NAL unit may include overhead information in the form of the NAL header in addition to the headers of the slices.